In the manufacture of semiconductor devices, a resist pattern formed on a semiconductor substrate is removed after an implant of ions has been carried out. The ion implantation, particularly when carried out at a high dose, causes the surface of the resist pattern to harden, and removal of such a resist pattern with a hardened surface cannot be done by using a usual cleaning acidic solution. Therefore, dry ashing with the help of a gas of O.sub.2 is used. This dry ashing technique, however, cannot achieve complete removal of every resist pattern formed on the semiconductor substrate.
Referring now to FIGS. 3a to 3f, one of the conventional semiconductor device fabrication techniques is described.
Formed on a semiconductor substrate 10 is a 1.5 .mu.m-thick resist layer 11 (see FIG. 3a). This resist layer 11 is formed by a novolac resin and naphthoquinonediazide (i.e. TSMR-V3, a product of Tokyo Ohka Kogyo Co., Ltd.) and is used for forming resist patterns for usual ion implantations.
As shown in FIG. 3b, an i-line stepper (NA: 0.50), not shown, gives off i-line indicated by arrows 13 so that the resist layer 11 is exposed to the i-line 13 through an ion implantation mask 12 having a predetermined pattern.
Next, the resist layer 11 is subjected to a heating process as well as to a developing process, and is formed into a resist pattern 11'.
As shown in FIG. 3d, with using the resist pattern 11' as a mask, an implant of As.sup.+ ions, indicated by arrows 14, is carried out to the semiconductor substrate 10 at a high dose of (1.times.10.sup.16). As described previously, such a high-dose ion implantation causes the surface of the resist layer's 11' to deteriorate or harden, and thus the resist pattern 11' cannot be removed by a sulfuric acid solution.
Therefore, a dry ashing process using O.sub.2, indicated by arrows 15, is performed to remove the resist pattern 11' (see FIG. 3e).
Although most of the resist pattern 11' are removed by the dry ashing process 15, the semiconductor substrate 11' still suffers from some unwanted residues/particles 16(see FIG. 3f).
The residues/particles 16 become a defect cause at later processing steps, thereby decreasing the yield of semiconductor devices.
In order to remove the residues/particles 16 lingering on the semiconductor substrate 10, the surface of the semiconductor substrate 10 can be rinsed using an acidic cleaning solution. However, this only worsens the already worsened throughput due to the previous dry ashing process, and process costs cannot be held low because the dry aching process and the cleaning process are additionally involved. Therefore, it is preferable not to use acidic cleaning solutions to remove a resist residue.